The present invention relates in general to self-propelled center pivot irrigation systems which are comprised of a plurality of self-propelled articulated span units which mount interconnecting sections of water carrying conduit, each such span unit having drive motor means and motor control means which regulate the operation of the drive motor means to maintain the span units in pre-determine relative alignment. In particular, the present invention relates to the control means which regulate the operation of the drive motor means.
The preferred relating alignment for the individual span units of the system is accomplished when the units are in essentially a straight longitudinal array with the individual units revolving about the center pivot of the system aligned in an imaginary true vertical plane revolving about an imaginary truly vertical line located co-axial with the systems center pivot. "True vertical" as used herein is the indication of vertical as provided by a so called "spirit" level or a plumbbob suspended on a plumb line, i.e. truly vertical as to the gravitational center of the earth.
The flexure junction between adjacent span units is provided with angularity sensor means providing an input to the motor control means. An early example of such apparatus is disclosed in the U.S. Pat. No. 3,394,729 to C. D. Bower et al wherein a cord is fastened at both ends to one span unit and passed around a shaft on the next adjacent unit. Changes in angular relationship between the two units cause the shaft to be rotated by the cord. As the shaft rotates, appropriate switch means are actuated to cause the desired signal input to the motor control means. As can be seen, such apparatus results in an linear response over the complete field of angularity movement between adjacent span units.
In later adaptations of such control apparatus, it has been common to connect a sensor arm to one span unit and to have the sensor arm operationally interact with a motor control actuator mechanism of an adjacent span unit. It has been found in these prior embodiments that the use of a rigidly mounted sensor arm measures changes in angularity in a plane perpendicular to the vertical plane of its unit only and can thus transmit spurious signals to the motor control actuator mechanism when the self-propelled irrigation system traverses an uneven terrain. An improvement in such apparatus is shown in copending United States patent application No. 731,695 now Pat. No. 4,085,771, entitled "Angularity Sensor Means For Center Pivot Irrigation System" by Edwin J. Hunter and assigned to the common assignee of this application. In such apparatus, provision is made for maintaining the sensing apparatus in a position normal to the earth's gravitational pull thereby making the angularity sensor responsive to changes in angularity between the span unit relative only to the true horizontal plane. While the apparatus of the afore-mentioned copending application has successfully met its desired objectives, the use of a rigid sensor arm as incorporated therein provides certain limitations as to switch placement within the motor control means particularly as wherein a more compact structure is desired.
More specifically, such motor control apparatus as employed in many articulated sprinkler systems employs a pair of switches connected in series between the motor and its source of power for each direction of travel. The motor control means of the span unit furthest from the center pivot is made to operate the motor means driving the span unit on a continuous basis. As the angularity between this unit and the next adjacent span unit increases, a first position will be reached wherein the two span units are out of angularity alignment tolerance. At this point, a first normally opened switch is closed causing the motor control means to actuate the motor means to drive the span unit next adjacent the outside unit to "catch-up" to the outside unit. When the second unit is within angularity tolerance, the normally opened switch is once again opened in response to changes in the angularity sensor means. This action continues seriatim from each span unit to the next adjacent span unit closer to the center pivot with each outbound unit acting as a master to the next more interior unit which respond as a slave unit to its master. Each motor control unit means additionally contains a second normally closed switch which is a safety switch. Should the angularity between any two adjacent span units increase beyond the first position to a second position wherein the angularity between the two units is deemed to have reached a safety limit, the angularity sensing means activates the second switch to open it and, thereby, disable power to all the span units.
Typically, in such apparatus, the aforementioned sensor arm is connected to an actuator rod which moves an actuator element longitudinally within the motor control means to actuate switches therein. The changes in angularity between adjacent span units required to cause actuation of the first switch position causing "catch-up" action to be taken is typically less than 1 degree. If spacing of the switches and corresponding dimensioning of the sensor arm and control arm are such as to prevent spurious actuation of the switch from normal movement of the span units over the terrain to be irrigated, the distance to be travelled by the switch actuator between the first position switch and the second position switch can become extraordinarily large. If the motor control means is adapted to operate in both directions i.e. forward and reverse, actuation switches and safety switches must be provided in both directions. The distance in such case between the two safety switches at extreme opposite ends of the travel of the switch actuator can be considerable. The housing for the motor control means containing such switches and actuator must, of course, be large enough to accommodate the length of travel of the components therein.
Wherefore, it is the object of the present invention to provide an improved motor control actuator being resistive to spurious signal generation while maintaining a relatively short distance of travel between actuating positions.